I have been publicly promoting social bookmarks since 2005, when I was using a defunct tool called FURL. Since then I have used Magnolia, Delicious, Diigo, and Pinboard. The first two are gone and the last two seem to be waning. For example, I cannot access my account settings when logged-in to Pinboard. Others are having no luck getting support from Diigo.
What are social bookmarks? They are like bookmarks on your browser except they are available online from any device, they are searchable, and you can add metadata like hashtags and categories. They can be public or private. The most important aspect is that they are shareable. Here are my Diigo bookmarks and Pinboard pins, as examples.